1. Field of the Invention
This invention relates to the reduction of stresses in a corrugated cable shield, and, more particularly, to a shielded cable capable of withstanding forces imparted to the shield when the cable is installed.
2. Prior Art
Filled service cable, in general, comprises a core having a plurality of pairs of individually insulated conductors in which the interstices between the conductors are filled with a waterproofing compound, and a polyester material which is wrapped about the core. By a process known as armoring, a corrugated metallic tape is wrapped longitudinally about the filled, wrapped core to form a shield having an overlapped seam with the corrugations of the overlapped seam portions being nested together. A vinly chloride polymer material is extruded over the shield to form a jacket having an outside diameter on the order of 0.280 to 0.350 inch. Such a cable is shown, for example, in U.S. Pat. No. 3,885,380 which issued on May 27, 1975 in the name of J. M. Hacker.
Typically, filled service cables extend underground between distribution cables and customers' premises and are installed by means of a plow having a vibratory cutting blade which is referred to as a plow share. As the plow share is vibrated, the cable is moved through a tube attached to the blade and into a trench along a curved path having a sharp radius which may be on the order of one to two inches. The corrugations of the shield on the outside of the bend in the cable undergo tension tending to flatten out the corrugations, while the corrugations on the inside are subject to compression thereby tending to cause cracking and separation of the cable shield. This may occur because the nested, registered corrugations restrict relative motion between the overlapping edges of the shield. Once the shield becomes cracked or separated, torsional shear and longitudinal forces which continue to be imparted to the cable during installation cause the jacket to thin down in the vicinity of each crack or separation and render the cable vulnerable to damage brought on by rodent attack or by mechanical forces imparted to underground cables.
The cable is prestressed during its manufacture when it may be twisted in a clockwise or counter-clockwise direction depending on the shield material, on the characteristics of the shield-forming apparatus, and on how it is moved off the manufacturing line. The stresses imparted to the cable during manufacture may be increased during its handling and installation. For example, in the field, a reel of cable is rested on the radial face of one of its flanges and the cable convolutions then pulled over the other flange. Since only the trailing end of the cable on the reel is accessible, the cable could be pulled off the reel in a manner which would impart a twist opposite to that direction of twist imparted during manufacture, thereby increasing the stresses in the cable.
In service cables made in accordance with the prior art, the overlapping nested portions of the corrugated shield may be moved in opposite circumferential directions since an amount of movement is possible between the overlapping shield portions and the plastic jacket without the plastic being torn or the plastic being torn from the shield. However, since the overlapping portions of the shield are nested and hence locked together, relative movement of the overlapping shield portions in the longitudinal direction is prevented and forces such as those experienced during installation cause cracking or separation of the shield.
The problem of degradation of the cable occurring during the installation of the cable can be avoided by manufacturing a cable having freedom of movement in a direction longitudinally of the cable between overlapping portions of the corrugated shield. Although this can be accomplished by forming the shield from a non-corrugated metallic tape, such a cable could not be flexed sufficiently along a sharp radius curved path such as those met in underground installations and could crack. The use of a jacket comprising a highly stress-resistant material such as, for example, polyethylene is also not a viable alternative because of the flame retardance requirements for jacketing materials of products used adjacent customers' premises.
Nowhere has the prior art recognized the problem of degradation of cables brought on by excessive stresses when overlapping nested corrugations of the shield cannot move relative to each other in a longitudinal direction as the cable is advanced in a sharp radius bend in an underground installation. At best, the prior art has recognized and dealt with the problem of seam separation during cooling of the cable jacket such as, for example in U.S. Pat. No. 3,272,912 where a polyethylene jacket shrinks and causes overlapping edge portions of a shield to slide relative to each other to reduce the original cross-sectional shape.